1. Field of the Invention
The present invention relates to an inverter device for driving and controlling a sensorless DC brushless motor, and to an air conditioner applying such an inverter device to a motor-driven compressor using a sensorless DC brushless motor as a driving source.
2. Description of the Related Art
The following explains an example of an air conditioner for a vehicle, which mounts a conventional motor-driven compressor using a sensorless DC brushless motor as a driving source, and which includes a battery or other DC power source.
FIG. 20 shows a system configuration of an air conditioner for a vehicle. In the drawing, reference numeral 101 is an air duct, and air is sucked in through an air inlet port 103 by an action of an indoor fan 102, and after heat exchange by an indoor heat exchanger 104, the air is blown out into a vehicle compartment from an air blowout port 105.
A refrigeration cycle is constructed by the indoor heat exchanger 104 together with a motor-driven compressor 106 using a sensorless DC brushless motor as a driving source, a four-way changeover valve 107 for changing flow of a refrigerant to select cooling or heating, a throttle device 108, and an outdoor heat exchanger 110 for exchanging heat with fresh air by an action of an outdoor fan 109 (motor).
Reference numeral 111 is an inverter device for operating a sensorless DC brushless motor as a driving source of the motor-driven compressor 106, and the operation thereof together with the indoor fan 102, four-way changeover valve 107 and outdoor fan 109, is controlled by an air conditioner controller 112.
The air conditioner controller 112 is connected with an indoor fan switch 113 for turning on or off the indoor blast and controlling the fan power, an air conditioner switch 114 for selecting cooling or heating, or turning off, a temperature control switch 115, and a communication device 116 for communicating with a vehicle controller.
In this system, for example, when the blast is turned on and low power is set by the indoor fan switch 113 and cooling is instructed by the air conditioner switch 114, the air conditioner controller 112 sets the four-way changeover valve 107 as shown by a solid line in the diagram, and the indoor heat exchanger 104 is used as evaporator and the outdoor heat exchanger 110 as condenser, and the outdoor fan 109 is turned on and the indoor fan 102 is set to be a low power.
According to the temperature control switch 115, by varying a rotating speed of the motor-driven compressor 106 using the inverter device 111, the temperature of the indoor heat exchanger 104 is adjusted. When cooling or heating is turned off by the air conditioner switch 114, the motor-driven compressor 106 and outdoor fan 109 are turned off.
When the indoor fan switch 113 is turned off, the indoor fan 102 is turned off, and the motor-driven compressor 106 and outdoor fan 109 are also turned off in order to protect the refrigeration cycle.
On the other hand, when an OFF command of cooling or heating operation is received from a vehicle controller (not shown) via the communication device 116 because of a reason of saving a power or protecting a battery, the air conditioner controller 112 conducts an action similar to turning off the cooling or heating operation conducted by the air conditioner switch 114.
FIG. 21 shows a motor-driven compressor having a sensorless DC brushless motor as an example of the conventional motor-driven compressor 106.
In the diagram, a compression mechanism 28, a motor 31 and others are installed in a metal casing 32.
The refrigerant is sucked in through a suction port 33, and when the compression mechanism 28 (scroll mechanism, in this example) is driven by the motor 31, the refrigerant is compressed. The compressed refrigerant passes through the motor 31 in the metal casing 32 and then cools the motor 31, and is then discharged from a discharge port 34. A terminal 39 connected to a winding of the motor 31 inside is connected to the inverter device 111 in FIG. 20.
In an air conditioner for a vehicle mounting such a motor-driven compressor, it is important to drive at a low noise and low vibration from the viewpoint of riding comfort and effects of vibration on other devices. Especially in an electric car, since there is no engine, the operation is very silent (in a hybrid electric car while running by a motor without starting engine), and further while stopping, the motor-driven compressor can be driven by a battery power source, and in this case since there is no running noise or vibration, the noise and vibration of the motor-driven compressor will be more noticeable.
However, a current feeding system by the inverter device 111 adapted to the conventional motor-driven compressor 106 is an 120-degree power feeding system, and a magnetic field change is an interval of 60 degrees (a current feeding in an interval of 60 degrees). For example, see Patent Document 1: Japanese Patent Laid-open Publication No. H8-163891, page 8, FIG. 4.
Accordingly, torque fluctuations are significant in the motor 31 for driving the compression mechanism 28, and it was difficult to lower the noise and vibration.
FIG. 22 shows a circuit example of a construction having the inverter device 111 and coupled with the motor portion of the motor-driven compressor. In the diagram, reference numeral 121 is a battery, 122 is inverter operation switching elements connected to the battery 121, and 123 are inverter operation diodes. Reference numeral 124 shows stator windings of the motor, and 125 shows a magnet rotor of the motor. Reference numeral 126 is a current sensor which detects a power supply current, calculates the power consumption, and protects the switching elements. Reference numeral 127 is a phase shift circuit for detecting a position of the magnet rotor 125 from a voltage of the stator windings 124, and 128 is a comparator. Reference numeral 129 is a control circuit for controlling the switching elements 122 on the basis of signals from the current sensor 126, comparator 128 and others.
On the other hand, in the case of a sinusoidal driving, since a permanent magnet rotor is driven by a continuous rotating magnetic field, torque fluctuations are small. Therefore, it is desired to use a sinusoidal driving inverter device which produces sinusoidal current. For detection of a position of the permanent magnet rotor, two current sensors are used for detecting the current of the stator windings. For example, see Patent Document 2: Japanese Patent Laid-open Publication No. 2000-333465, page 9, FIG. 2.
FIG. 23 shows another circuit example using the inverter device 111. As compared with the construction in FIG. 22, the comparator 128 and phase shift circuit 127 are not provided, but there are further provided a current sensor 130 for detection of U-phase current and a current sensor 131 for detection of W-phase current in order to detect the position of the magnet rotor 125 from the current of the stator windings. The control circuit 129 calculates the current of the other phase from the current values of two phases from the two current sensors (two current sensors are needed, but any two phases of the phases U, V, W will do), detects the position of the magnet rotor 125, and controls the switching elements on the basis of the signals from the current sensor 126 and others.
The current sensor 130 for detection of U-phase current and current sensor 131 for detection of W-phase current are provided on the inverter output lines of which the potentials are always changing due to on/off application of a voltage of the battery 121, and therefore a photo coupler or the like is needed for signal transmission to the control circuit 129. As a result, the current sensors are complicated in structure, and a simple structure only by a shunt resistance can not be realized.
Aside from the low noise and low vibration, the air conditioner for a vehicle is also demanded to be small in size and light in weight from the viewpoint of accommodation and running performance.